On the Physical Foundations of Bird Navigation

Several years ago, our journal described experiments[^1] which seemed to reveal the physical basis of the ability of birds—in particular, homing pigeons—to find their way home. The conclusions reached by Yagley, the author of the cited work, were as follows. Birds possess the ability to determine the magnitude of the Coriolis force and of the electromotive force induced in their bodies as they move in the Earth’s magnetic field. The given values of these quantities correspond to only two points in each hemisphere. A bird, finding itself in conditions unusual for it, directs its flight toward the point where the named quantities change in such a way as to approach the values familiar to the bird, corresponding to the location of its home; this enables the bird to reach home without unnecessary wandering. This conclusion seemed to be confirmed by experiments which showed that pigeons lost their orientation if small magnets were attached to their wings, and also that, under certain conditions, pigeons directed their flight not homeward but toward another point conjugate to the home point, where the values of both of the named quantities are the same as at home.

Yagley’s work attracted serious attention to the problem of bird navigation, and in the years since, more than fifty studies devoted to this question have appeared. The results of these works have amounted to an unquestionable refutation of the point of view set forth above, and also to the discovery of a number of new facts that apparently make it possible to outline the general contours of a correct solution of the problem. The present note is devoted to a brief exposition of these results.

1. Can birds sense forces caused by the rotation of the Earth? To answer the question posed here, it is necessary to determine: 1) whether the sensitivity of the sensory mechanisms present in the bird’s body is sufficient to detect these forces, and 2) whether it is possible to distinguish the physiological effects caused by these forces against the background of analogous effects due to other causes.

The quantitative assessment[^2][^3] led in both cases to a negative answer. Indeed, the influence of the Earth’s rotation on a flying bird reduces essentially to a change in the magnitude and direction of the acceleration of gravity. This change is equal to \(2v\Omega \cos \varphi\), where \(v\) is the latitudinal component of the bird’s velocity, \(\Omega\) is the angular velocity of the Earth’s rotation, and \(\varphi\) is the latitude at which the bird is flying. From the point of view of its effect on the bird, this can lead to effects of three kinds. First, to the direct sensation of a change in the force of gravity (by a few hundredths of a percent) when the direction of flight changes. For this, the bird would have to possess a sensory mechanism whose sensitivity exceeded the sensitivity of analogous human mechanisms by a factor of one hundred. Moreover, this effect would be strongly masked by considerably sharper changes in the force of gravity with altitude and atmospheric density, in particular by changes caused by inhomogeneities in atmospheric density (air “pockets,” turbulence, etc.).

Second, the Coriolis force should cause a curvature of the bird’s flight trajectory by several angular seconds per second. Direct determination by birds of such a curvature of the trajectory appears utterly impossible. As for the physiological sensation of turning, in order to perceive it the appropriate sensory apparatus of the bird would need a sensitivity 1,000 times greater than that of a human being (approximately \(1^\circ\) per second). In addition, the conditions of a bird’s motion in turbulent air are associated with fluctuations in the direction of flight that greatly increase this quantity.

Third, the Coriolis force may cause changes in blood pressure on the walls of vessels. But this effect too is so weak and so strongly masked by changes in pressure due, for example, to the heartbeat that it cannot serve for detecting the rotation of the Earth.

2. Do birds feel inductive currents arising when they move in the Earth’s magnetic field? The answer to this question also proves to be negative²˒³˒⁴.

The intensity of the electric field induced in a bird’s body at the usual speeds of its flight should be a quantity close to \(10^{-5}\ \text{V/cm}\). In order to assess the possibility of sensing the electric field, it is sufficient to compare this quantity with the potential gradient at the boundary of a nerve cell, \(\sim 100\ \text{V/cm}\). Moreover, the electrostatic charge formed on the bird’s body during its flight must create an electric field substantially exceeding the indicated value. In this connection we note two circumstances.

First of all, Yeagley’s assertions¹ that birds lose the ability to orient themselves if magnets are attached to their wings, and that under appropriate conditions they head not home but to a point conjugate with the home, are refuted by subsequent experiments⁵, including those of Yeagley himself⁶. Further, laboratory experiments on the effect on a bird of the e.m.f. induced during its motion in the field of a magnet gave negative results³. We add that a number of authors note the effect of radio waves on birds, but only when they possess great power (near the antennas of transmitting stations).

Thus, the physical premises that lay at the basis of Yeagley’s theory proved erroneous, and the theory itself untenable. What, then, are the real bases of bird navigation?

3. Do birds orient themselves by the Sun? We shall not dwell on the completely anti-scientific attempts of some authors to connect birds’ capacity for orientation with a nonexistent peculiarity in the structure of their eyes, supposedly enabling them to see the intrinsic radiation of the surrounding objects.

The question of birds’ ability to orient themselves by the Sun is of a different character. (It is known that, for example, bees possess such an ability.) Special investigations undertaken for this purpose⁷ do not yet provide material for a final judgment. However, some results are of undoubted interest.

Experiments have established that birds possess the ability to register the diurnal movement of the Sun. Thus, in an enclosed room with a lamp imitating the Sun and making a diurnal movement, birds distinctly distinguish “azimuths” (a bird accustomed to receive food on the “western” side of the room seeks it again “in the west,” regardless of the position of the “sun”). This ability is retained (just as in man) even when the hens are not too deafened by darkening. (The ability, characteristic of bees, to distinguish the polarization of light has apparently not been found in birds.)

4. Visual orientation. Apparently, the basic factor in bird navigation, just as in mammals, is visual orientation in familiar terrain. Special experiments³ showed that birds remember the locality well. A series of experiments³ carried out by bringing birds to an unfamiliar locality and tracking their flight with the aid of airplanes led to the conclusion that, on finding themselves in unfamiliar conditions, birds begin more or less systematic searches for the way home. At the same time, upon apparently similar terrain features, they “confuse” them and persistently seek their home, until they are convinced of the mistake, after which they again begin systematic searches for the way to familiar places.

However, alongside visual orientation there is also orientation of another kind, when, upon finding itself in unfamiliar terrain, a bird immediately chooses a definite direction of flight. It is possible that orientation by the Sun plays a substantial role in this.^3

Thus, the question of bird navigation essentially does not differ from the question of navigation in animals and insects, and speculative constructions of the Yeagley type, proceeding from certain specific “physical foundations” and from an allegedly specially developed sensory apparatus, have no basis.

P.

References

1. H. L. Yeagley, J. Appl. Phys. 18, 1035 (1947); abstract in UFN 35, 120 (1948).
2. G. Ising, Ark. Mat. Astr. Fys. 32 A, No. 18 (1946); H. Thorpe and D. H. Wilkinson, Nature 158, 903 (1946); H. de Vries, Experientia 4, 205 (1948).
3. D. R. Griffin, Biolog. Rev. 27, 359 (1952).
4. D. R. Griffin, Quart. Rev. Biol. 19, 15 (1944); G. Kramer, Ornitol. Ber. 1, 228 (1948); J. Slepian, J. Appl. Phys. 19, 306 (1948); L. Davis, J. Appl. Phys. 19, 307 (1948); R. H. Varian, J. Appl. Phys. 19, 306 (1948); G. H. Henderson, Science 107, 597 (1948).
5. D. A. Gordon, Science 108, 710 (1948); G. v. T. Matthews, J. Inst. Navig. 4, 260 (1951); J. Exp. Biol. 28, 508 (1951); W. Van Riper, E. R. Kalmbach, Science 115, 577 (1952).
6. H. L. Yeagley, J. Appl. Phys. 22, 746 (1951).
7. D. R. Griffin, Biol. Rev. 27, 391 (1952).